Internal combustion engines that run with an A/F ratio significantly leaner than stoichiometric ratio are being developed, and some have already been commercially implemented.
For example, there is an apparatus disclosed in Japanese Unexamined Patent Publication No. H07-158462. In the apparatus disclosed therein, when an accelerating condition is detected by an acceleration detecting means, engine boost pressure is increased while maintaining a lean A/F ratio. However, in the apparatus disclosed in the cited patent document, the engine is operated with a lean A/F ratio only within the range indicated as a region 2 in FIG. 6 in the cited patent document, and is not operated with a lean A/F ratio in the high-speed, high-load range.
Further, Japanese Unexamined Patent Publication No. H03-23327 discloses an internal combustion engine that runs with a lean A/F ratio in the high-speed, high-load range, but the engine is designed to operate at stoichiometry in the low-to-mid speed, low-to-mid load range.
On the other hand, from the standpoint of saving energy, there is a need for engines that consume less fuel. However, with the internal combustion engine disclosed in the above patent document 1, as the engine is not operated with a lean A/F ratio in the high-speed, high-load operating range, fuel consumption is high in the high-speed, high-load driving range. Conversely, with the internal combustion engine disclosed in the above patent document 2, as the engine is designed to operate at stoichiometry in the low-to-mid speed, low-to-mid load range, fuel consumption is high in the low-to-mid speed, low-to-mid load operating range.